Karl Unruh

Growth of germanium-carbon alloys on silicon substrates by molecular beam epitaxy

Metastable Ge1−yCy alloys were grown by molecular beam epitaxy as homogeneous solid solutions having a diamond lattice structure. The substrates were (100) oriented Si wafers and the growth temperature was 600 °C. We report on measurements of the composition, structure, lattice constant, and optical absorption of the alloy layers. In thick relaxed layers, C atomic fractions up to 0.03 were obtained with a corresponding band gap of 0.875 eV. These alloys offer new opportunities for fundamental studies, and for the development of silicon‐based heterostructure devices.

Band gap bowing and refractive index spectra of polycrystalline AlxIn1−xN films deposited by sputtering

The AlGaInN semiconductor system is currently of high interest for applications in blue light emitting devices. AlInN is a prospective material for lattice matched confinement layers. We measure the refractive index as well as the band gap across the entire compositional range of high-quality polycrystalline AlInN samples. Strong band gap bowing is observed.

Structure and stability of sputter deposited beta‐tungsten thin films

The structure and stability of thin tungsten films prepared by radio frequency magnetron sputter deposition have been studied by x‐ray diffraction and x‐ray photoelectron spectroscopy. The structure of these films has been found to systematically evolve from the metastable A15 β‐W phase to the equilibrium A2 α‐W phase with decreasing oxygen impurity concentration. Within the β‐W phase a decrease in the concentration of incorporated oxygen results in a monotonic decrease in the lattice parameter of the unit cell until the β‐W phase eventually becomes unstable, and the α‐W phase is formed.

High temperature soft magnetic materials: feco alloys and composites

We have systematically investigated the microstructural effects including grain size, precipitation, and structural order parameter on the high temperature magnetic and mechanical properties of FeCo-based commercial alloys. At high temperatures the equilibrium nonmagnetic precipitates significantly deteriorate the soft magnetic properties. Poor mechanical properties are mainly due to the nature of the ordered structure of FeCo alloys. Based on this understanding we have designed new magnetic composites by reinforcing FeCo alloys with high strength fibers. The magnetic and mechanical properties can thus be improved independently through optimizing the magnetic matrix and fiber network, respectively. These new magnetic composites show excellent soft magnetic and mechanical properties. In particularly, negligible creep has been observed at 600/spl deg/C.

Effect of composition on thermal stability and electrical resistivity of Ta-Si-N films

The role of composition on the resistivity and thermal stability of sputtered Ta–Si–N films have been studied using X-ray diffraction, Rutherford backscattering spectrometry, and sheet resistance measurement. Films with higher silicon to tantalum ratio were found to be more thermally stable and have higher sheet resistance than films with lower Si to Ta ratio. While Ta0.28Si0.07N0.65 starts to crystallize at about 900°C, Ta0.24Si0.10N0.66 and Ta0.24Si0.12N0.64 were thermal for heat treatment below 1100°C. In-situ sheet resistance measurement also showed that the sheet resistance for the alloys varies with composition and decreases with temperature. Our results indicate that Ta–Si–N films would find other applications in semiconductor devices, beside being used as a diffusion barrier.

Giant magnetoresistance studies in (Fe,Co)‐Ag films

Giant magnetoresistance (GMR) values were measured in thin films of Fe and Co in Ag. The best GMR results were observed in Ag‐rich specimens, with maxima of 25% (30 K) and 14% (20 K) observed in Co20Ag80 and Fe25Ag75, respectively. Magnetic data indicate a spin glasslike behavior in the as‐deposited Ag‐rich films. The as‐made samples have a nanostructure, with a face centered cubic structure. Annealing of the samples over the temperature range of 200 °C to 700 °C led to grain growth and subsequent phase separation of the constituent metals. A summary of the magnetic and electrical transport properties is presented, in relation to the crystal structure and microstructure of the TM‐Ag films (TM=Fe,Co).

Optical and electronic properties of SiGeC alloys grown on Si substrates

Abstract Metastable Si 1 − x − y Ge x C y alloys were grown by molecular beam epitaxy on (100) Si substrates. Solid elemental sources were used for the Si and Ge beams, and a resistively heated graphite filament was used for the C beam. Up to 3 at% of C was incorporated in the alloy layers. Optical transmission measurements showed that the absorption edge of thick layers increased to higher energies with increasing C fraction, and revealed the presence of SiC and GeC vibrational modes in the infrared. At low temperatures, the alloys showed significant photoluminescence. The bandgap energies of thick layers increased linearly with the C fraction and followed a linear dependence of the bandgap on composition. Measurements of the valence band density of states using X-ray photoelectron spectroscopy indicated that the valence band energy maximum increased with the C fraction relative to that of SiGe alloys of similar composition. Our results indicated that SiGeC alloys are promising materials for Si-based heterostructure devices.

The effects of oxidation temperature on the capacitance–voltage characteristics of oxidized AlN films on Si

The thermal oxidation of AlN thin films produces a high quality insulator which exhibits the gate voltage-controlled charge regimes of accumulation, depletion, and inversion on Si surfaces. The temperature dependence of oxidation is important for device processing. We report on the composition, structure, and electrical properties of the AlN versus the oxidization temperature. AlN layers 500 nm thick were deposited by rf sputtering on p-type Si (100) substrates, followed by oxidation in a furnace at temperatures from 800 to 1100 °C with O2 flow. An oxidation time of 1 h produced layers of Al2O3 with small amounts of N having a thickness of 33 nm at 800 °C, and 524 nm at 1000 °C. Electrical measurements of metal-oxide-semiconductor capacitors indicated that the dielectric constant of the oxidized AlN was near 12. The best layer had a flatband voltage near zero with a net oxide trapped charge density less than 1011 cm−2. These results show that oxidized AlN has device-grade characteristics for the gate regi...

THERMALLY OXIDIZED ALN THIN FILMS FOR DEVICE INSULATORS

The structural, optical, and electronic properties of an insulating material prepared by the thermal oxidation of AlN thin films on Si have been studied by a number of different experimental techniques. The thermal oxidation at 1100 °C of reactively sputtered AlN films on Si wafers was found to result in the formation of an oxide with a relative Al to O concentration near Al2O3 with small amounts of incorporated N. The structure of the AlO:N oxide could be varied between amorphous and polycrystalline, depending on the preparation conditions, and the oxide surface was found to be approximately three time smoother than the as-sputtered AlN films. Metal–oxide–silicon capacitors had an oxide charge density of about 1011 cm−2, capacitance–voltage characteristics similar to pure SiO2, and a dielectric constant of 12.4. Infrared measurements yielded a refractive index of 3.9. These results indicate that thermally oxidized AlN films show promise as insulating structures for many integrated circuit applications, p...

High frequency properties of polymer composites consisting of aligned Fe flakes

We have successfully fabricated composites consisting of aligned Fe flakes embedded in a polymer matrix. Similarly high values of permeability and permittivity have been achieved, resulting in potentially significant improvements in device miniaturization, impedance matching, and bandwidth. Various effective medium theory models fail to give accurate values of permeability and permittivity of flake/polymer composites, indicating that modifications are needed for flake shape inclusions.